This invention relates to a closed loop air/fuel ratio controller for an internal combustion engine.
It is generally known that the amount of hydrocarbons, carbon monoxide and oxides of nitrogen present in the exhaust gases emitted from an internal combustion engine may be substantially reduced by controlling the air/fuel ratio of the mixture supplied to the engine and catalytically treating the exhaust gases emitted therefrom. For example, by controlling the air/fuel ratio of the mixture supplied to the engine near the stoichiometric value, a catalytic converter of the three-way type may be utilized to oxidize the carbon monoxide and hydrocarbons and reduce the oxides of nitrogen. The optimum air/fuel ratio of the mixture supplied to the engine for maximizing the conversion of the before-mentioned exhaust gas constituents is substantially the stoichiometric ratio. If the air/fuel ratio deviates from this ratio, either the reducing or oxidizing efficiency of the catalytic converter decreases resulting in an increase of the corresponding exhaust gas constituent.
In view of the foregoing, it is desirable to provide for the control of the air/fuel ratio of the mixture supplied to the engine substantially constant at the optimum ratio for maximizing the conversion efficiency of the three-way converter. To achieve this result, the control of the air/fuel ratio is provided by a closed loop controller which senses the condition of the exhaust gases and controls the ratio of air and fuel mixture supplied to the engine in response to the sensed condition so as to achieve substantially a stoichiometric ratio.
Two significant factors exist, however, which render it difficult to control the air/fuel ratio at a substantially constant value at the stoichiometric ratio. First, typical exhaust gas sensors used in closed loop air/fuel controllers are generally characterized in that they provide an output voltage that shifts abruptly between a high value representing a rich mixture relative to the stoichiometric value and a low level output representing a lean mixture relative to the stoichiometric value. Consequently, the sensor output is generally useful to indicate only the sense of deviation of the air/fuel ratio relative to the stoichiometric value. The second factor is the transport delay between the time at which a particular air and fuel mixture is supplied to the internal combustion engine and the time at which the resulting condition of the exhaust gases is sensed by the exhaust gas sensor. The tendency of this transport delay in the closed loop air/fuel ratio control system is to cause the air/fuel ratio controller to overshoot the desired stoichiometric air/fuel ratio which results in a decrease in either the oxidizing or reducing efficiency of the three-way catalytic converter.
It would be desirable to provide for a closed loop air/fuel ratio controller in which the variations in the controlled air/fuel ratio are minimized so as to maximize the conversion efficiency of the three-way catalytic converter.